Okay, this is going to sound weird.

I have been messing around with my Edge for about a month. I have probably programmed the PCM 50 times trying new things to see what happens. (And I have learned a lot in the process. Some pretty weird stuff happens when you try odd combinations of settings!)

In the end, for every day use, for me, the default level 2 tune is best.

So I put the truck back to stock, and started the process of programming a default level 2 tune. I then got the idea to make it custom just to see what defaults were put in the fields on each screen.

They were all as expected, and even the gear ratio and tire diameter was right.

But every time I hit "ok" for the tire diameter, and then went back in to verify the tire diameter was right, it increased a few mm. I started at 2312 (stock) and ended up at 2322 after a few checks. At that point it stopped increasing.

So, I decided to just load the level 2 tune with no customization.

What is the chance that the default level 2 tune will have my correct gear ratio and tire diameter?

It seemed to pick it up okay when I checked.

So after I programmed it, I thought, If I program it again, and load the defaults, then I can see what it is actually programmed at.

So I look and everything looks okay, except the tire diameter, after a few checks, starts increasing again.

I figure, no problem, I'll take the key out, "reboot" the Edge, and check again.

The tire diameter is still 2322 even though it should be loading the default that is in the PCM.

So I remove the Edge from the OBDII port and plug it back in. It is still 2322.

So, finally, how can I verify that my currently programmed PCM has 2312 and 3.55 in it?

(Other than all that, the Edge rocks!)

 

Here's the problem with the tire size....

Because we are calculating in millimeters and the PCM uses revs per mile, there is a slight variance in input values versus stored values. It takes a few mm to equal 1 rev/mi. The PCM doesn't support decimal values or fractions, so the rev/mi values are rounded to the next whole number. This is fine for the first time you change the tire size value, but every time you go back in and check, the Evo needs to take the current rev/mi value and convert it back into a mm value. This calculation doesn't always come out evenly, so you end up with another fraction that is rounded up which in turn ends up with another fraction going out... and so on and so on. I know this is a pain in the butt, but I didn't write that part of the code so I just deal with it.

They only way to truly verify that the value you put in is the value in the PCM is by verifying that your speedo is correct. Other than that, you'll have to readjust the tire size each time you go in to check it.

Keep in mind that 25.4 mm = 1 inch. 1 inch of circumference = .3183 diameter (just under 1/3") which isn't a whole lot. When looking at 2312 mm circumference, 25.4mm = a 1.1% difference, which is roughly 1 MPH at 100 MPH or 1/2 MPH at 55 MPH... again not much, especially when you consider that the speedo is 2 MPH fast to begin with!

Hope this clears that up for you.

Bill

 

Thanks Bill, for a clear explanation.

Mine did this too, adding 3 mm each time I went back to the the tire size. I had intended to ask you about it, but then things got sort of "busy" for you and I forgot (I don't adjust the settings on the Edge everyday).

What you describe makes sense, and I agree (since I'm a programmer too) that the code could have been written a bit differently. Too bad you didn't do it!

- Jack

 

Bill - You recommend a 5% "squish", correct?

 

I used 3% and my Edge and GPS are dead on. The speedo is still 2 fast but this is normal

 

I just marked my tire with chalk and drove forward to get two full passes. I then measured that spots on the pavement where the chalk was and got my number. Edge manual said 2600 and the actual was 2527. Thats about a 3% difference.

 

In general, 5% for "squish" comes out about right. Of course, tire compound, tread pattern, and inflation pressure can all have an effect on the accuracy. The best way to do it is the mbogosia did... chalk it, roll it (I usually do 3 times), chalk it again, and measure. This will give you a much better representation of what your tire is doing when you drive.

Take care.

Bill